1. Field of the Invention
The present invention relates generally to fluid compressors. It has particular, but not exclusive, application to compressors used for low-compression-ratio vapor compression.
2. Background Information
Among the many systems that use fluid compressors are vapor-compression distillers. Vapor-compression distillers generally make use of centrifugal compression because of its simplicity, cost-effectiveness, and reasonable efficiency. But that efficiency tends to degrade as distiller capacity is scaled downward. Below, say, 25 gallons of distillate per hour, efficiency considerations tend to favor reciprocating compressors, at least theoretically.
All other factors being equal, a lower compression ratio makes a vapor-compression distiller more efficient. But a reciprocating compressor""s complexity tends to introduce efficiency-compromising factors such as leakage paths and the energy expense associated with check-valve operation.
I have found a way of reducing these effects and thus contributing to a reciprocating compressor""s efficiency. According to my invention, piston rings mounted on the compressor""s pistons seal to the wall of the piston chamber, as is normal for piston rings, but the rings are free to move axially with respect to their respective pistons to some extent. The piston rings are arranged so that the across-the-piston pressure difference that results when the piston is driven in the downstream direction tends to move the piston ring axially into a position in which it additionally seals to the piston. It thus becomes relatively effective at driving fluid in the downstream direction and thereby compressing it. When the piston moves in the upstream direction, though, the piston ring tends to be come unseated from the piston and thereby leave a fluid path around it.
In short, the piston ring additionally acts as a check valve. This eliminates not only the need for a separate check valve but also the need for the springs that check valves typically include. Since the same element serves as both the check valve and the piston ring, there is less opportunity for leakage flow than there would be in systems that employ separate elements. The invention thus contributes to efficiency and mechanical simplicity.